Structure-Induced Switching of the Band Gap, Charge Order, and Correlation Strength in Ternary Vanadium Oxide Bronzes.

Recently, V2 O5 nanowires have been synthesized as several different polymorphs, and as correlated bronzes with cations intercalated between the layers of edge- and corner- sharing VO6 octahedra. Unlike extended crystals, which tend to be plagued by substantial local variations in stoichiometry, nanowires of correlated bronzes exhibit precise charge ordering, thereby giving rise to pronounced electron correlation effects. These developments have greatly broadened the scope of research, and promise applications in several frontier electronic devices that make use of novel computing vectors. Here a study is presented of δ-Srx V2 O5 , expanded δ-Srx V2 O5 , exfoliated δ-Srx V2 O5 and δ-Kx V2 O5 using a combination of synchrotron soft X-ray spectroscopy and density functional theory calculations. The band gaps of each system are experimentally determined, and their calculated electronic structures are discussed from the perspective of the measured spectra. Band gaps ranging from 0.66 ± 0.20 to 2.32 ± 0.20 eV are found, and linked to the underlying structure of each material. This demonstrates that the band gap of V2 O5 can be tuned across a large portion of the range of greatest interest for device applications. The potential for metal-insulator transitions, tuneable electron correlations and charge ordering in these systems is discussed within the framework of our measurements and calculations, while highlighting the structure-property relationships that underpin them.

Contrasting 1D tunnel-structured and 2D layered polymorphs of V2O5: relating crystal structure and bonding to band gaps and electronic structure.

New V2O5 polymorphs have risen to prominence as a result of their open framework structures, cation intercalation properties, tunable electronic structures, and wide range of applications. The application of these materials and the design of new, useful polymorphs requires understanding their defining structure-property relationships. We present a characterization of the band gap and electronic structure of nanowires of the novel ζ-phase and the orthorhombic α-phase of V2O5 using X-ray spectroscopy and density functional theory calculations. The band gap is found to decrease from 1.90 ± 0.20 eV in the α-phase to 1.50 ± 0.20 eV in the ζ-phase, accompanied by the loss of the α-phase's characteristic split-off dxy band in the ζ-phase. States of dxy origin continue to dominate the conduction band edge in the new polymorph but the inequivalence of the vanadium atoms and the increased local symmetry of [VO6] octahedra results in these states overlapping with the rest of the V 3d conduction band. ζ-V2O5 exhibits anisotropic conductivity along the b direction, defining a 1D tunnel, in contrast to α-V2O5 where the anisotropic conductivity is along the ab layers. We explain the structural origins of the differences in electronic properties that exist between the α- and ζ-phase.

Atomic Origins of Monoclinic-Tetragonal (Rutile) Phase Transition in Doped VO2 Nanowires.

There has been long-standing interest in tuning the metal-insulator phase transition in vanadium dioxide (VO2) via the addition of chemical dopants. However, the underlying mechanisms by which doping elements regulate the phase transition in VO2 are poorly understood. Taking advantage of aberration-corrected scanning transmission electron microscopy, we reveal the atomistic origins by which tungsten (W) dopants influence the phase transition in single crystalline WxV1-xO2 nanowires. Our atomically resolved strain maps clearly show the localized strain normal to the (122̅) lattice planes of the low W-doped monoclinic structure (insulator). These strain maps demonstrate how anisotropic localized stress created by dopants in the monoclinic structure accelerates the phase transition and lead to relaxation of structure in tetragonal form. In contrast, the strain distribution in the high W-doped VO2 structure is relatively uniform as a result of transition to tetragonal (metallic) phase. The directional strain gradients are furthermore corroborated by density functional theory calculations that show the energetic consequences of distortions to the local structure. These findings pave the roadmap for lattice-stress engineering of the MIT behavior in strongly correlated materials for specific applications such as ultrafast electronic switches and electro-optical sensors.

Is educational attainment associated with the onset and outcomes of low back pain? a systematic review and meta-analysis.

BACKGROUND: Low back pain (LBP) is the leading global cause of years lived with disability. Of the biopsychosocial domains of health, social determinants of LBP remain under-researched. Socioeconomic status (SES) may be associated with the onset of new LBP or outcomes of acute LBP, with educational attainment (EA) being a key component of SES. The association between EA and LBP has yet to be the subject of a dedicated review and meta-analysis. PURPOSE: To review evidence of the association between EA and a) onset or b) outcomes of acute and subacute LBP in the adult general population and to conduct statistical pooling of data where possible. METHODS: An electronic search was conducted in MEDLINE, Embase, CINAHL, and ProQuest from inception to 2nd November 2023 including reference lists to identify relevant prospective studies. Risk of bias (RoB) was assessed using the Quality in Prognostic Studies (QUIPS) tool. Where adequate data were available, estimates were pooled using a random-effects meta-analysis. Overall evidence for each outcome was graded using an adapted GRADE. RESULTS: After screening 8498 studies, 29 were included in the review. Study confounding and attrition were common biases. Data from 19 studies were statistically pooled to explore EA as a predictor of new LBP onset or as prognostic for outcomes of acute or subacute LBP. Pooled results showed no association between EA and the onset of new LBP (OR: 0.927, 95%CI: 0.747 to 1.150; I2 = 0%). For predicting outcomes of acute LBP, compared to those with no more than secondary-level education, post-secondary education or higher was associated with better outcomes of pain (OR: 0.538, 95%CI: 0.432 to 0.671; I2 = 35%) or disability (OR: 0.565, 95%CI: 0.420 to 0.759; I2 = 44%). High heterogeneity (I2>80%) prevented meaningful pooling of estimates for subacute LBP outcomes. CONCLUSION: We found no consistent evidence that lower EA increases the risk of LBP onset. Lower EA shows a consistent association with worse LBP outcomes measured at least 3 months later after acute onset with inconclusive findings in subacute LBP. Causation cannot be supported owing to study designs. High-quality research is needed on potential mechanisms to explain these effects.

Reversible interconversion of a divalent vanadium bronze between δ and ß quasi-1D structures.

Charge fluctuations along the quasi-1D frameworks of M(x)V(2)O(5) bronzes have evinced much recent interest owing to the manifestation of colossal metal-insulator transitions and superconductivity. Depending upon the nature of the intercalating cation (M), distinctive geometries of the V(2)O(5) framework are accessible. Herein, we demonstrate an unprecedented reversible transformation between double-layered (δ) and tunnel (ß) quasi-1D geometries for nanowires of a divalent vanadium bronze, Ca(x)V(2)O(5) (x ≈ 0.23), upon annealing-induced dehydration and hydrothermally induced hydration. Such a facile hydration/dehydration-induced interconversion between two prominent quasi-1D structures (accompanied by a change in charge-ordering motifs) has not been observed in the bulk and is posited to result from the ease of propagation of crystallographic slip processes across the confined nanowire widths for the δ → ß conversion and the facile diffusion of water molecules within the tunnel geometries for the ß â†’ δ reversion.

Synthesis, characterization, and finite size effects on electrical transport of nanoribbons of the charge density wave conductor NbSe3.

NbSe(3) exhibits remarkable anisotropy in most of its physical properties and has been a model system for studies of quasi-one-dimensional charge density wave (CDW) phenomena. Herein, we report the synthesis, characterization, and electrical transport of single-crystalline NbSe(3) nanoribbons by a facile one-step vapour transport process involving the transport of selenium powder onto a niobium foil substrate. Our investigations aid the understanding of the CDW nature of NbSe(3) and the growth process of the material. They also indicate that NbSe(3) nanoribbons have enhanced CDW properties compared to those of the bulk phase due to size confinement effects, thus expanding the search for new mesoscopic phenomena at the nanoscale level. Single nanoribbon measurements of the electrical resistance as a function of temperature show charge density wave transitions at 59 and 141 K. We also demonstrate significant enhancement in the depinning effect and sliding regimes mainly attributed to finite size effects.

Transformers: the changing phases of low-dimensional vanadium oxide bronzes.

In this feature article, we explore the electronic and structural phase transformations of ternary vanadium oxides with the composition MxV2O5 where M is an intercalated cation. The periodic arrays of intercalated cations ordered along quasi-1D tunnels or layered between 2D sheets of the V2O5 framework induce partial reduction of the framework vanadium atoms giving rise to charge ordering patterns that are specific to the metal M and stoichiometry x. This periodic charge ordering makes these materials remarkably versatile platforms for studying electron correlation and underpins the manifestation of phenomena such as colossal metal-insulator transitions, quantized charge corrals, and superconductivity. We describe current mechanistic understanding of these emergent phenomena with a particular emphasis on the benefits derived from scaling these materials to nanostructured dimensions wherein precise ordering of cations can be obtained and phase relationships can be derived that are entirely inaccessible in the bulk. In particular, structural transformations induced by intercalation are dramatically accelerated due to the shorter diffusion path lengths at nanometer-sized dimensions, which cause a dramatic reduction of kinetic barriers to phase transformations and facilitate interconversion between the different frameworks. We conclude by summarizing numerous technological applications that have become feasible due to recent advances in controlling the structural chemistry and both electronic and structural phase transitions in these versatile frameworks.

Emptying and filling a tunnel bronze.

The classical orthorhombic layered phase of V2O5 has long been regarded as the thermodynamic sink for binary vanadium oxides and has found great practical utility as a result of its open framework and easily accessible redox states. Herein, we exploit a cation-exchange mechanism to synthesize a new stable tunnel-structured polymorph of V2O5 (ζ-V2O5) and demonstrate the subsequent ability of this framework to accommodate Li and Mg ions. The facile extraction and insertion of cations and stabilization of the novel tunnel framework is facilitated by the nanometer-sized dimensions of the materials, which leads to accommodation of strain without amorphization. The topotactic approach demonstrated here indicates not just novel intercalation chemistry accessible at nanoscale dimensions but also suggests a facile synthetic route to ternary vanadium oxide bronzes (M x V2O5) exhibiting intriguing physical properties that range from electronic phase transitions to charge ordering and superconductivity.